Phytoavailability of lead altered by two Pelargonium cultivars grown on contrasting lead-spiked soils

Purpose

This study assesses the potential of two contrasted fragrant Pelargonium cultivars to induce pH and dissolved organic carbon (DOC) changes in the soil solution, Pb speciation, and their subsequent effects on rhizosphere phytoavailable Pb.

Materials and methods

Rooted plantlets were grown in special devices, floating on aerated nutrient solution in PVC tanks. This setup allows roots to be physically separated, through a mesh, from a 3-mm soil matrix layer that can be considered as rhizosphere soil. Two contrasted soils, each spiked with Pb-rich particles, emitted from a battery recycling industry, were used at total burdens of 500 and 1500 mg Pb kg^?1 in addition to a control unspiked soil. Soil solution pH, phytoavailable Pb, DOC, Pb adsorption, precipitation on roots, and Pb phases in soil and plant were investigated.

Results and discussion

Attar of Roses (Attar) cultivar acidified its rhizosphere by 0.4 pH units in both spiked soils. Concolor Lace (Concolor) was unable to change soil solution pH on soil-1 and increased it by 0.7 units on soil 2. Concentrations of Pb in soil solution from Attar plants were always higher than those of Concolor ones. DOC contents of both unspiked soil-1 and soil-2 without plants were not significantly different. In the case of spiked samples, DOC contents in the rhizosphere soil were increased by three and two times for Attar and Concolor, respectively, compared to the unspiked soil without plant. Both cultivars were able to increase DOC contents, independent of soil type and level of contamination. Accumulation of Pb in shoots and roots was higher in Attar as compared to Concolor due to enhanced available Pb as a result of pH and DOC modifications of the rhizosphere soil. Significant amounts of Pb were adsorbed on roots of both cultivars. X-ray elemental analysis of precipitates on roots revealed the association of Pb with P in cylinder-like structures. Extended X-ray absorption fine structure (EXAFS) spectroscopy revealed that Pb was present, to a major extent in the inorganic form, mainly as PbSO₄ in the soil, whereas it was complexed with organic species within plant tissues. The conversion of Pb into organic species could decrease toxicity, may enhance plant tolerance, and could increase translocation.

Conclusions

Plant-induced changes were responsible for the modification of lead phases within the soil. Immobile forms present in the source leaded particles as well as in the soils were converted into soluble species, ultimately improving the phytoavailable or soil solubilized Pb.

     

Zinc speciation and isotopic exchangeability in soils polluted with heavy metals

Correct characterization of heavy metal availability is a prerequisite for the management of polluted soils. Our objective was to describe zinc (Zn) availability in polluted soils by measuring the isotopic exchangeability of Zn in soil/solution ( E value) and in soil/plant systems ( L value), by assessing the transfer of Zn and ⁶⁵Zn in the fractions of a six-step selective sequential extraction (SSE) in incubated soils and by identifying Zn forms in soils by means of extended X-ray absorption fine structure (EXAFS) spectroscopy. We distinguished three pools of exchangeable Zn: the pool of Zn exchangeable within 1 minute, which is observed in all soils, Zn exchangeable in the medium term, and the slowly and not exchangeable Zn. The amount of Zn present in the first two pools was similar to the L value measured with Thlaspi caerulescens . The first three steps of the SSE solubilized the first pool and a fraction of the second pool. Most of the second pool and a fraction of the third pool were extracted in the fourth step of the SSE, while the rest of the third pool was extracted in the final steps of the SSE. The EXAFS study conducted on two soils showed that more than half of the Zn was present in species weakly bound to organic compounds and/or outer sphere inorganic and organic complexes. Other species included strongly sorbed Zn species and Zn species in crystalline minerals. The EXAFS study of selected SSE residues showed that the specificity and the efficiency of the extractions depended on the properties of the soil studied.